This invention relates to semiconductor devices, particularly to those having trenches formed in their substrate, and more particularly to trenched semiconductor devices of improved immunity to destruction. The trenched semiconductor devices according to the invention include, but are not limited to, an insulated-gate bipolar transistor (IGBT) and insulated-gate field-effect transistor (IGFET).
Japanese Unexamined Patent Publication No. 2005-57028 is hereby cited as teaching an IGBT that is trenched for a higher breakdown voltage. As illustrated in FIG. 1 for a better understanding of the features of the instant invention, the prior art trenched IGBT has a plurality of trenches 2 in its semiconductor substrate 1′. The substrate 1′ is variously doped to include n-type emitter regions 3, p-type base region 4, n-type base region 5, n+-type buffer region 6, p+-type collector region 7, p−-type reduced surface field (RESURF) region 8, and n+-type channel stop region 9. The trenches 2 receive gate conductors 11 as gate electrodes via gate insulators 10. An emitter electrode 12 is received in part in recesses 33 and 34 in one major surface 21 of the substrate 1′, in part overlies the trenches 2 via insulators 36, and is electrically coupled directly to both n-type emitter regions 3 and p-type base region 4. A collector electrode 13 underlies the other major surface 22 of the substrate 1′ and adjoins the p+-type collector region 7.
In order to turn this prior art IGBT on, a gate voltage may be applied to make the gate conductors 11 higher in potential than the emitter electrode 12, with the collector electrode 13 held higher in potential than the emitter electrode. The result is the creation of a channel in the p+-type base region 4 adjacent the trenches 2, permitting a current flow from collector electrode 13 to emitter electrode 12. The prior art IGBT is turned off by making the gate conductors 11 less in potential than the threshold. Thereupon the channel will disappear from the p-type base region 4. During the ensuing off period, the voltage between collector electrode 13 and emitter electrode 12 will be higher than that during the on period. A relatively high reverse bias voltage will be impressed between p-type base regions 4 and n-type base region 5, and a depletion layer or depletion region 14′ will spread as indicated by the broken line in FIG. 1.
The contour of the depletion region 14′ depends upon the relative positions of the trenches 2, which may be thought of as comprising a series of relatively inside trenches 2a and an outside trench 2b. The depletion region 14′ will be contoured mostly as desired in the neighborhoods of the inside trenches 2a, successfully mitigating field concentrations. Adjacent the outside trench 2b, however, the depletion region will not spread so much as desired because it has a neighboring trench on one side only, rather than on both sides like each of the inside trenches 2a. The field intensity will therefore become higher there than in the vicinities of the inside trenches. Breakdown has therefore been easy to occur near this outmost trench, with the consequent flow of a concentrated large current eventually leading to the destruction of the IGBT in the worst case.
An obvious expedient for alleviation of field intensity adjacent the outmost trench might seem to make the p-type base region 4 deeper on the outside of that trench. Such deeper part of the base region 4 would serve to spread the depletion region deeper for mitigation of field concentration adjacent the outmost trench. This measure is objectionable because the diffusion of a p-type dopant is the only possible way of deepening the required part of the base region 4. The deeper diffusion of the p-type dopant would inevitably invite its lateral (horizontal) diffusion in addition to that in the depth (vertical) direction. The base region 4 would therefore become larger in its surface area, necessitating use of a correspondingly larger substrate.
The above discussed problems arising from trenches are not limited to IGBTs. Similar difficulties have been encountered with other types of trenched semiconductor devices notably including IGFETs.